This invention relates generally to radar altimeters, and more specifically to, methods and apparatus for calibration of multiple antenna radar altimeters.
Interferometric synthetic aperture radar altimeters are used to provide precision terrain navigation. The precision terrain navigation is provided, at least in part, by measuring a cross track angle to the highest ground point or radar target that reflects radar energy back to the radar altimeter. Typically, the radar energy is transmitted from the radar altimeter in a cross track doppler swath below the aircraft on which the radar altimeter is incorporated. In one system, the cross track angle is measured by processing phase differences or time of arrival differences of radar return signals from the target at three separate antennas precision spaced and positioned on a baseline passing through the pitch axis of the aircraft.
Angular accuracy of typically on the order of fractions of a milli-radian is required for such precision terrain navigation. A radar processor which receives the radar return signals from the three antennas will provide such milli-radian accuracy if an exact measurement of antenna spacing and positioning is provided, and if exact cable delay and processing differences through the three radar channels to the radar processor is known. However, even with very careful assembly of an antenna assembly which includes the three radar antennas, the resulting errors in the cross track angle, or radar measured angle, are more than can be tolerated by a navigation system. Factors in the antenna assembly which can cause such errors include, for example, measurement and subsequent cutting of the cabling utilized in the assembly which connect the three antennas to the radar processor, and routing of the cables within the antenna assembly.
Present calibration methods utilize ground radar reflectors, whose exact positions are known through surveying, to provide known target positions to be used to calibrate the aircraft mounted radar altimeter during a flight test. The flight test calibration method, suitable for testing experimental data collection systems such as used in the development of radar terrain elevation maps, is far too costly for a production radar environment where several hundred radar altimeters might typically be produced each month.
An apparatus for calibrating an interferometric radar altimeter is provided. The radar altimeter provides an interferometric angle to a target based on radar energy received at a right antenna, a left antenna, and an ambiguous antenna located between the right and left antennas. The apparatus comprises a turntable on which the radar is mounted, a turntable controller which controls positioning of the radar altimeter on the turntable, a radar energy source receiving transmit signals from the radar altimeter, a reflector, and a calibration unit. The reflector reflects radar energy from the radar source towards the radar altimeter on the turntable and collimates the radar energy reflected to the turntable. The calibration unit receives an angle from the turntable controller indicative of a position of the radar altimeter and a measured angle from the radar altimeter indicative of a perceived orientation of the radar altimeter with respect to the collimated radar energy. The calibration unit calculates a radar calibration correction based on a difference between the angle received from the turntable and the measured angle received from the radar altimeter. The calibration unit then provides the calibration correction to the radar altimeter. The radar altimeter can then be installed in a vehicle.
In another aspect, a method for determining discrepancies between an orientation of a radar altimeter and a measurement of the orientation by the radar altimeter is provided. The radar altimeter has a plurality of receiving antennas, and the discrepancies are caused by varying delays in processing the radar returns received at each receiving antenna. The method comprises mounting the radar altimeter in a known orientation, directing a collimated radar signal towards the radar altimeter, providing a radar measurement of an angle of the radar altimeter with respect to the collimated radar signal, and determining a difference between an angle representative of the known orientation and the radar measurement angle.
In still another aspect, a method for compensating a measured angle to a radar target provided by a radar altimeter is provided. The radar altimeter includes a plurality of receive antennas hard mounted to a chassis of the radar altimeter. The method comprises receiving a known orientation of the radar altimeter with respect to an expected radar return, directing a collimated radar signal towards the radar altimeter, receiving a radar measured angle resulting from the collimated radar signal, determining a difference between an angle representative of the known orientation and the radar measured angle, and providing radar calibration correction to the radar altimeter, the corrections causing the radar measured angle to be equal to the angle representative of the known orientation of the radar altimeter.
In yet another embodiment, a calibration unit receiving a radar measured angle and a turntable angle is provided. The turntable angle is indicative of an orientation of a radar altimeter with respect to a collimated radar signal. The calibration unit comprises a software code segment for calculating radar calibration corrections based on a difference between the two received angles.